This invention relates to an improved method for chlorinating aluminous, oxygen-containing material in the presence of a reducing agent.
Chlorination of aluminous, oxygen-containing materials in the presence of a reducing agent, such as carbon, to produce aluminum chloride is well known. Various methods have been devised to improve the extent or quality of chlorination, but possible use of superatmospheric pressures is one approach that has not been explored to any great extent until the present. Pressure has been used to condense aluminum chloride vapors to promote melting of aluminum chloride (French Pat. No. 334,132), to force chlorine gas through molten aluminum (Brooks U.S. Pat. No. 1,165,065) and to form briquettes from coking coal and alumina or bauxite (McAfee U.S. Pat. No. 1,217,471). It was not recognized, however, until the present invention, that chlorinating an aluminous material in a reactor maintained at pressures greater than three atmospheres would have the advantages of both increasing the mass capacity of equipment by the factor F, ##EQU1## and increasing the degree of approach to complete gas conversion at constant gas-solids contact time. One explanation for this failure to seriously consider use of pressures greater than three atmospheres is that thermodynamic studies conducted at atmospheric pressure indicated that the reaction rate dependence on chlorine pressure was less than first order. Since a reaction which is less than first order with respect to chlorine pressure would be expected to yield a product at a rate-to-pressure ratio which diminishes with increasing pressure, those skilled in the art did not consider use of superatmospheric pressures as a worthwhile means for improving aluminum chloride production.
Russell et al U.S. Pat. No. 3,842,163 is one example which illustrates that some in the art who had considered use of superatmospheric pressures did not believe that increased reactor pressure would improve chlorination efficiency. While stating that pressures between 0.1 and 10 atmospheres may be used, the Russell et al patent indicates that chlorination is generally carried out at about one atmosphere, with pressures of 1-3 atmospheres described as preferred. While Russell et al stated that higher pressures permit a greater throughput which normally offsets any decrease in efficiency, operation at pressures higher than three atmospheres was not considered a sufficiently attractive procedure such that we have been able to find reports of actual trials in the literature.
Atherholt (U.S. Pat. No. 2,048,987) used pressures of two to three atmospheres to promote the reaction's approach to equilibrium and thereby cause a greater amount of aluminum chloride per pound of chlorine to be formed. Atherholt failed to realize, however, that by increasing mass flow through a reactor maintained at pressures greater than three atmospheres, the rate-to-pressure ratio would increase.